Kaolin clay was one of the first inorganic materials to be dried commercially on a large scale using spray dryers. To prepare pure kaolin for the paper, paint, plastics, rubber, and ceramic industries, crude kaolin from a mine is milled, crushed, and then slurried with water and chemical dispersants. A fine clay slip containing about 25% solids is formed. The slip is flocculated with acid, further chemically treated to improve its quality, centrifuged and then filtered to obtain a slurry of about 60% solids content. In conventional processing the 60% slurry is spray dried to produce a particulate product containing less than one percent free moisture and about 14% crystal bound water.
Two main products dominate the current kaolin product market. The first is a dry particulate product and the second is a near 70% solids content kaolin slurry. Individual customer preference for these products depends on factors which are not relevant to the present invention. However, it is essential that a kaolin supplier be capable of producing near 70% slurries and dry products at minimum cost.
Since the production of these products is accomplished by removing water from the kaolin clay slurries, and since this is accomplished by heating and evaporating water from the raw product, the amount of heat required to operate such a process is of primary concern to manufacturers of these products. Therefore, a number of different techniques have been previously considered to save energy.
U.S. Pat. No. 1,746,294 discloses a process for the continuous calcination of gypsum wherein waste heat from a preheater furnace may be put into a steam jacket surrounding a calcinating chamber or used to heat steam supplied to the jacket.
U.S. Pat. No. 4,246,039 discloses a kaolin clay process wherein a suspension of hydrated kaolin fed to an electrofilter is preheated by direct heat exchange with hot gases from a clay calciner used to remove water of hydration from the kaolin.
It is also known to utilize calciner off gases to supply heat to a spray dryer. For example, U.S. Pat. No. 3,776,688 describes the operation of a rotating kiln plant for producing cement according to a wet process. The capacity of the kiln is increased, and heat economy improved, by drying part of the cement slurry in a spray dryer and introducing the spray dried material into the kiln. The spray drying is performed using exhaust gases from the kiln in an integrated operation.
U.S. Pat. No. 2,815,292 discloses a method for dewatering clay wherein waste gases from a dryer are delivered to a heat exchanger for heating a cold clay slip prior to dewatering.
U.S. Pat. No. 4,642,904 describes a process for drying a clay slurry which includes: dewatering an aqueous clay slurry by filtration, contacting the partially dewatered slurry with hot drying gases in an evaporative dryer, condensing part of the evaporated water and using the condensate to preheat aqueous clay slurry before and after being filtered.
U.S. Pat. No. 4,687,546 describes a method for concentrating a kaolin slurry whereby a beneficiated clay slurry is concentrated by evaporating water from the slurry by passing the slurry through one or more non-contact evaporative heat exchangers.
U.S. Pat. Nos. 4,717,559 and 4,962,279 disclose a kaolin calciner waste heat and feed recovery system wherein hot exhaust gas from the calciner is added to and delivered with a hot air stream from a combustor to a spray drier. Hot gases from the spray dryer may also be used to heat water in a scrubber and the heated water is used to heat the feed to a filtration process.
Notwithstanding the various efforts and techniques to conserve energy in the production of concentrated slurries and spray dried particulate products, as will be more fully described below, none of the prior techniques considered the process and apparatus of the present invention or provided the degree of savings and enhanced production of the present invention.